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I recently edited the Wikipedia article 50 Move Rule as follows:

The rationale for this rule is that pawn moves and captures are the only '''irreversible''' types of moves. That is, after each pawn move or capture, it is impossible for any previous position in the game to be repeated.

Unfortunately, my edit was quickly removed with the comment: This is unreferenced. Come up with a reference if you can.

I think this is pretty clear just by thinking about it, but is there any "formal proof" published anywhere that what I tried to add to the article is true?

EDIT: The words "the only" should be deleted. In addition to captures and pawn moves, losing the right to castle on either or both sides (by moving a rook or king), or failing to capture a certain pawn "en passant" (thus giving up the right to do so later) are also irreversible. These logically should also reset the 50-move count but are omitted from the rules for reasons of simplicity.

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  • For which part of what you tried to add are you seeking proof of its truth? That those are the only sorts of irreversible moves, or that that is really the rationale for the rule? Either way, your title is a little misleading and might be best reworded to indicate which sort of proof you are looking for.
    – ETD
    Commented Feb 22, 2015 at 17:19
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    Castling and declining to capture en-passant are also irreversible moves. In chess problems, the 50-move rule counts castling as a way of resetting the counter, see Codex at saunalahti.fi/~stniekat/pccc/codex.htm
    – phs
    Commented Feb 22, 2015 at 19:42
  • If you want a moment of glory editing the Wikipedia entry for the 50 move rule then you should know that this phrase in the current article is wrong - "Theoretically, a game could continue indefinitely this way". The introduction of the 75 move rule (a 75 move equivalent to the 50 move rule which allows the arbiter to intervene and declare the game a draw) in the July 1 2014 amendments means that in practice the game would not continue indefinitely even if both players were incalcitrant (neither player prepared to accept a draw) or ignorant (players don't know the rules).
    – Brian Towers ♦
    Commented Feb 23, 2015 at 7:15
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    The question sounds like you want references to the fact that what you wrote about irreversibility is the rationale for this rule, but your comments to Tony Ennis's anwer below seem to indicate that you want a proof that what you wrote about irreversibility is true. Which one do you want?
    – JiK
    Commented Feb 23, 2015 at 18:20
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    Yeah, you're right... I think I need both
    – StaleMate
    Commented Feb 23, 2015 at 18:38

3 Answers 3

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The two irreversible moves can be proved irreversible (it is impossible to replicate that board state later in the game) like so:

Assumptions: - Pawns can't move backward. - Every pawn move moves it forward. - Pieces can't be added to the board. - There is no way to create a pawn (unlike queens, bishops, knights, and rooks which can be created via pawn promotion).

Firstly, captures can be proved irreversible because they always remove a piece from the board. If a board state has N pieces and one is removed you now have N-1 pieces. There is no way to increase the number of pieces so you can never return to N pieces.

Secondly, since every pawn move moves it forward, a pawn can never return to its original position. The only way to have a pawn in that position later on is to have it be occupied by another pawn. But for another pawn to occupy the position that pawn has to move. There is only a finite number of pawns so after every pawn has moved to fill the position left by the previous pawn there are no pawns left to fill the position of the last pawn, preventing the board state from being reached again.

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  • I hope this gets accepted. +1!
    – user45266
    Commented Nov 2, 2018 at 5:07
  • @user45266 The question was asked 3 years ago, it's probably not going to be accepted haha.
    – Omegastick
    Commented Nov 2, 2018 at 5:07
  • Hey, you never know. Nice answer though!
    – user45266
    Commented Nov 2, 2018 at 5:11
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The only 'formal proof' you need is a reference to the rule book of the appropriate governing body. In the US, this could be the USCF. For international events, it would be FIDE.

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    I'm using the word "proof" in a mathematical sense
    – StaleMate
    Commented Feb 23, 2015 at 0:20
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    My point is that there is no 'proof'. There is an arbitrary rule. This rule is well defined.
    – Tony Ennis
    Commented Feb 23, 2015 at 2:50
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    Tony is absolutely right. Chess is a human construct and its rules a social convention. There are many different variants and there is no compulsion to play according to FIDE or USCF rules unless you are playing in one of their tournaments. There can be no proof in the sense of pure mathematics because the game and the rules are contingent.
    – Brian Towers ♦
    Commented Feb 23, 2015 at 10:44
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    I am looking for a formal proof that captures and pawn moves are irreversible.
    – StaleMate
    Commented Feb 23, 2015 at 13:47
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    That part is arbitrary, agreed. But not the "pawn moved or piece captured" part.
    – StaleMate
    Commented Feb 23, 2015 at 17:50
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Whoever gave the reason they did for removing your edit was being kind. Your supposed rationale for the rule is not correct. The 50 move rule and the 3 fold repetition rule are there to cut short games which are going nowhere.

If there are captures or pawn moves then the nature of the position is changing and there is the possibility of one side or the other winning or losing via these moves. In blocked positions where such moves are either impossible for either side or suicidal it makes no sense to just go on pushing wood. Irreversibility has nothing whatsoever to do with it.

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    "Your supposed rationale for the rule is nonsense." This seems too harsh to me. I agree completely with your statement that, "If there are captures or pawn moves then the nature of the position is changing and there is the possibility of one side or the other winning," but I would argue that it is exactly the irreversibility of such moves, as noted by the OP, that makes them signs of progress. ... (cont'd)
    – ETD
    Commented Feb 22, 2015 at 18:02
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    (cont'd) ... Players can shuffle pieces around forever (ignoring repetition for the moment), but there is a clear finite limit on the number of pawn moves and captures that can be made (because these are irreversible), and that is why such moves makes sense as a measure of progress occurring in the game.
    – ETD
    Commented Feb 22, 2015 at 18:03
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    The comments can be understood in relation to the 75 move rule (9.6), if not to the 50 move rule. Two players who conspire to have an infinite game against an arbiter who wants it to be finite, will end up drawn after less than something like 130 * 75 moves. This does relate to reversibility, but also to an intentionally low number of irreversible moves that the rule recognizes as a proof of progress.
    – Jirka Hanika
    Commented Feb 13, 2016 at 23:16

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